Over the past decade, there has been a concerted effort to reduce atmospheric pollution caused by volatile solvents which are emitted during the painting process. However, it is often difficult to achieve high quality, smooth coating finishes, such as are required in the industrial and automotive industry, without using organic solvents which contribute greatly to flow and leveling of a coating. In addition to achieving near-flawless appearance, such coatings must be durable and abrasion resistant, yet economical and easy to apply.
The use of conventional thermally curable thermosetting resins for coating compositions can be undesirable because of the necessity for thinning the resins with solvents to provide easy application to substrates. The use of solvents is undesirable for cost and environmental reasons because the solvents must be removed from the coating composition during cure.
I. Azuma et al., “Acrylic Oligomer for High Solid Automotive Top Coating System Having Excellent Acid Resistance”, Progress in Organic Coatings 32 (1997) 1–7, disclose acrylic oligomers containing siloxy groups synthesized using trimethylsiloxyethyl methacrylate, which have lower polarity and lower viscosity than acrylic oligomers comprising unblocked hydroxyl groups and are useful for formulating high solids automotive top coatings.
U.S. Pat. No. 6,045,870 discloses an organic solvent-based heat-curable coating composition comprising: (A) a carboxyl-containing vinyl polymer or carboxyl-containing polyester compound having an acid value of 50 to 500 mg KOH/g wherein 20 mol % or more of the carboxyl groups are silylated; (B) at least one epoxide selected from (B-1) an epoxy-, hydroxyl- and hydrolyzable alkoxysilyl-containing vinyl polymer, (B-2) the vinyl polymer (B-1) wherein 20 mol % or more of the hydroxyl groups are silylated hydroxyl groups, and (B-3) an epoxy compound having a number average molecular weight less than 1,000; (C) a crosslinked particulate polymer, and optionally (D) a reactive organopolysiloxane, the composition having a solid content of 65 weight % or more and having good acid and scratch resistance, low temperature curability, storage stability and recoat adhesion.
Color-plus-clearcoating systems involving the application of a colored or pigmented basecoat to a substrate followed by application of a transparent or clearcoat over the basecoat have become increasingly popular as original finishes for a number of consumer products including, for example automotive vehicles. The color-plus-clearcoating systems have outstanding appearance properties such as gloss and distinctness of image, due in large part to the clearcoat. Such color-plus-clearcoating systems have become popular for use with automotive vehicles, aerospace applications, floor coverings such as ceramic tiles and wood flooring, packaging coatings and the like.
Topcoat film-forming compositions, particularly those used to form the transparent clearcoat in color-plus-clearcoating systems for automotive and industrial applications, are subject to defects that occur during the assembly process as well as damage from numerous environmental elements. Such defects during the assembly process include paint defects in the application or curing of the basecoat or the clearcoat. Damaging environmental elements include acidic precipitation, exposure to ultraviolet radiation from sunlight, high relative humidity and high temperatures, defects due to contact with objects causing scratching of the coated surface, and defects due to impact with small, hard objects resulting in chipping of the coating surface.
Typically, a harder more highly crosslinked film may exhibit improved scratch resistance, but it is less flexible and much more susceptible to chipping and/or thermal cracking due to embrittlement of the film resulting from a high crosslink density. A softer, less crosslinked film, while not prone to chipping or thermal cracking, is susceptible to scratching, waterspotting, and acid etch due to a low crosslink density of the cured film.
Further, elastomeric automotive parts and accessories, for example elastomeric bumpers and hoods, are typically coated “off site” and shipped to automobile assembly plants. The coating compositions applied to such elastomeric substrates are typically formulated to be very flexible so the coating can bend or flex with the substrate without cracking. To achieve the requisite flexibility, coating compositions for use on elastomeric substrates often are formulated to produce coatings with lower crosslink densities or to include flexibilizing adjuvants which act to lower the overall film glass transition temperature (Tg). While acceptable flexibility properties can be achieved with these formulating techniques, they also can result in softer films that are susceptible to scratching. Consequently, great expense and care must be taken to package the coated parts to prevent scratching of the coated surfaces during shipping to automobile assembly plants.
Despite recent improvements in color-plus-clearcoating systems, there remains a need in the automotive coatings art for topcoats having low volatile organic content (“VOC”) and which are capable of being formulated into high solids coating compositions. Other desirable attributes of the coating include good recoat adhesion and good scratch resistance without embrittlement of the film. Moreover, it would be advantageous to provide topcoats for elastomeric substrates utilizable in the industrial and automotive industries which are both flexible and resistant to scratching.